1. Field of the Invention
The invention relates to a full-bridge circuit, and in particular to a full-bridge circuit employed in a direct-current fan.
2. Brief Discussion of the Related Art
FIG. 1a is a block diagram of a conventional heat dissipation system. FIG. 1b shows a circuit of the full-bridge circuit in FIG. 1a. As shown in FIG. 1a, the heat dissipation system comprises a fan 10, a control circuit 11, and a full-bridge circuit 12. The full-bridge circuit 12 comprises switches Q1 to Q4 and an induction coil L. When the switches Q1 and Q4 are turned on, the switches Q2 and Q3 are turned off and a current path of an input voltage Vin includes the switch Q1, the induction coil L, and the switch Q4. Conversely, when the switches Q1 and Q4 are turned off, the switches Q2 and Q3 are turned on and a current path of the input voltage Vin includes the switch Q2, an induction coil L, and the switch Q3.
The switches Q1 to Q4 of the full-bridge circuit 12 are respectively controlled by four control signals output from node A1 to A4 of the control circuit 11. The control signals from the nodes A1 and A4 are in phase and out of phase with the control signals from the nodes A2 and A3. To ensure that the switch Q3 is turned off when the switch Q1 is turned on, a high-level control signal from the node A1 turns on the switch Q1 and a switch S1. Thus residual current of the switch Q3 is grounded, preventing false operation of the switch Q3. Similarly, a high-level control signal from the node A2 turns on the switch Q2 and a switch S2. Thus, that residual current of the switch Q4 is grounded, preventing false operation of the switch Q4.
The control circuit 11 is typically a micro-processor control unit (MCU) for outputting the four control signals A1 to A4. As the largest output voltage of a MCU is 5V, the lower bridge, the switches Q3 and Q4, of the full-bridge circuit 12 must substantially composed of low voltage elements, such as 4.5V metal oxide semiconductor field effect transistors (MOSFETs). The low voltage elements however have weak anti-noise ability resulting in false operation due to floating voltage in the circuit. As the number of available low voltage elements is limited, effective cost reduction is difficult. Moreover, four control signals are required from a MCU to control switches Q1 to Q4 of the full-bridge circuit 12, resulting in complicated circuitry.
Thus, a full-bridge circuit employed by a heat dissipation system for improving anti-noise ability and simplification of the full-bridge circuit are desirable.